1. Field of the Invention
This invention relates to case magnesium alloys of low porosity and fine grain size and their method of production.
2. Description of Prior Art
Magnesium alloys are widely used engineering materials, for example, as cast parts in the aerospace and automobile industries. The magnesium alloys have a low specific gravity, typically about 1.8 g/cm.sup.3, and their light weight and relatively high strength provide a high strength-to-weight ratio.
Particularly advantageous properties of magnesium alloys include low density, high strength-to-weight ratio, good castability, easy machinability and good damping characteristics.
Magnesium alloy parts can be produced by the conventional casting methods including diecasting, sand casting, permanent and semi-permanent mold casting, shell casting and investment casting.
An especially useful class of magnesium alloy for cast parts, is that based on magnesium with minor amounts of aluminum and zinc and optionally manganese as alloying additions.
Magnesium alloys have experienced a rapid growth in use in cast parts in the automobile industry as a result of general requirements for lighter weight automobiles to conserve energy.
A particular disadvantage of the cast magnesium alloys is their relatively coarse grain size, typically about 225 .mu.m which results in reduction of tensile strength, ductility and pressure tightness of the alloy part as compared with a hypothetical magnesium alloy of fine grain size. The coarse grain size also results in an undesirable microporosity.
A further disadvantage, which seems to be especially characteristic of the aforementioned magnesium alloys based on magnesium, aluminum and zinc and optionally manganese, is the tendency of the alloy part to develop shrinkage microporosity during casting.
The conventional approach to reducing shrinkage microporosity in alloys involves the use of risers in combination with chilling during casting to promote directional solidification towards the riser whereby the shrinkage microporosity shifts towards the riser which forms a removable appendix to the main portion of the casting. In the case of the magnesium alloys, however, it is difficult to produce magnesium alloy parts free of shrinkage microporosity on a consistent basis, especially when the alloy solidifies over a long freezing range, which is the case with the aforementioned alloys based on magnesium, aluminum, zinc and manganese.